Chalcogen-containing compound, its preparation method and thermoelectric element comprising the same

Abstract

A chalcogen-containing compound of the following Chemical Formula 1 which exhibits excellent phase stability at a temperature corresponding to the driving temperature of a thermoelectric element, and also exhibits an excellent thermoelectric performance index (ZT) through an increase in a power factor and a decrease in thermal conductivity, a method for preparing the same, and a thermoelectric element including the same:
V.sub.1-xM.sub.xSn.sub.4-yPb.sub.yBi.sub.2Se.sub.7-zTe.sub.z  [Chemical Formula 1]
In the above Formula 1, V is a vacancy, M is an alkali metal, x is greater than 0 and less than 1, y is greater than 0 and less than 4, and z is greater than 0 and less than or equal to 1.

Claims

1. A chalcogen-containing compound represented by the following Chemical Formula 1:
V.sub.1-xM.sub.xSn.sub.4-yPb.sub.yBi.sub.2Se.sub.7-zTe.sub.z  [Chemical Formula 1] wherein, in the above Formula 1, V is a vacancy, M is an alkali metal, x is greater than 0 and less than 1, y is greater than 0 and less than 4, and z is greater than 0 and less than or equal to 1, wherein the chalcogen-containing compound has a face-centered cubic crystal lattice structure, the Se is filled in an anion site of the face-centered cubic lattice structure, the Sn, Pb, and Bi are filled in a cation site of the face-centered cubic lattice structure, the Pb is substituted by replacing a part of the Sn, the Te is substituted by replacing a part of the Se, the M is filled in at least some of vacant sites excluding the sites filled with Sn, Pb, Bi, Se, and Te in the face-centered cubic lattice structure, and the V is a vacant site of the remaining cationic sites.

2. The chalcogen-containing compound of claim 1, wherein the M is at least one alkali metal selected from the group consisting of Li, Na, and K.

3. The chalcogen-containing compound of claim 1, wherein the V, M, Sn, Pb, and Bi are randomly distributed at the site of (x, y, z)=(0, 0, 0), and Se and Te are randomly distributed at the site of (x, y, z)=(0.5, 0.5, 0.5).

4. The chalcogen-containing compound of claim 1, wherein the x+y+z is greater than 0 and less than or equal to 5.

5. The chalcogen-containing compound of claim 1, wherein the compound is selected from the group consisting of V.sub.0.8Na.sub.0.2Sn.sub.3.2Pb.sub.0.8Bi.sub.2Se.sub.6.95Te.sub.0.05, V.sub.0.8Na.sub.0.2Sn.sub.3.2Pb.sub.0.8Bi.sub.2Se.sub.6.6Te.sub.0.4, and V.sub.0.8Na.sub.0.2Sn.sub.3.2Pb.sub.0.8Bi.sub.2Se.sub.6.2Te.sub.0.8.

6. A method for preparing the chalcogen-containing compound of claim 1, represented by the following Chemical Formula 1:
V.sub.1-xM.sub.xSn.sub.4-yPb.sub.yBi.sub.2Se.sub.7-zTe.sub.z  [Chemical Formula 1] wherein, in the above Formula 1, V is a vacancy, M is an alkali metal, x is greater than 0 and less than 1, y is greater than 0 and less than 4, and z is greater than 0 and less than or equal to 1, comprising the steps of: mixing raw materials of Sn, Pb, Bi, Se, Te, and an alkali metal (M) and subjecting the mixture to a melting reaction; heat-treating the resultant product obtained through the melting reaction; pulverizing the resultant product obtained through the heat treatment; and sintering the pulverized product, wherein the mixing of raw materials is carried out by mixing the raw materials such that the molar ratio of Sn, Pb, Bi, Se, Te, and an alkali metal (M) is a ratio corresponding to 4-y:y:2:7-z:z:x.

7. The method for preparing the chalcogen-containing compound of claim 6, wherein the melting is carried out at a temperature of 700° C. to 800° C.

8. The method for preparing the chalcogen-containing compound of claim 6, wherein the heat treatment is carried out at a temperature of 550° C. to 640° C.

9. The method for preparing the chalcogen-containing compound of claim 6, further comprising a step of cooling the result of the heat treatment step to form an ingot between the heat treatment step and the pulverization step.

10. The method for preparing the chalcogen-containing compound of claim 6, wherein the sintering step is carried out by a spark plasma sintering method.

11. The method for preparing the chalcogen-containing compound of claim 6, wherein the sintering step is carried out at a temperature of 550° C. to 700° C. under a pressure of 10 MPa to 100 MPa.

12. A thermoelectric element comprising the chalcogen-containing compound according to claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a phase stability diagram of typical Sn—Bi—Se based chalcogen-containing compounds.

(2) FIG. 2 is a graph showing the results of X-ray diffraction analysis of the chalcogen-containing compound powder just before the sintering step in Examples 1 to 3, Comparative Example 1, and Reference Example.

(3) FIG. 3 is a graph showing the results of X-ray diffraction analysis of the chalcogen-containing compound powders of Comparative Examples 2 to 4.

(4) FIG. 4 is a graph showing the results of X-ray diffraction analysis after the sintered body finally produced through the sintering step in Examples 1 to 3, Comparative Example 1, and Reference Example is slowly cooled and left to stand at room temperature.

(5) FIG. 5 is a graph showing the results of measuring electrical conductivity versus temperature of the chalcogen-containing compounds in Examples 1 to 3, Comparative Example 1, and Reference Example.

(6) FIG. 6 is a graph showing the results of measuring the Seebeck coefficient versus temperature of the chalcogen-containing compounds in Examples 1 to 3, Comparative Example 1, and Reference Example.

(7) FIG. 7 is a graph showing the results of measuring the power factor versus temperature of the chalcogen-containing compounds in Examples 1 to 3, Comparative Example 1, and Reference Example.

(8) FIG. 8 is a graph showing the results of measuring the total thermal conductivity versus temperature of the chalcogen-containing compounds in Examples 1 to 3, Comparative Example 1, and Reference Example.

(9) FIG. 9 is a graph showing the results of calculating the lattice thermal conductivity versus temperature of the chalcogen-containing compounds in Examples 1 to 3, Comparative Example 1, and Reference Example.

(10) FIG. 10 is a graph showing the results of calculating the thermoelectric performance index versus temperature of the chalcogen-containing compounds in Examples 1 to 3, Comparative Example 1, and Reference Example.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(11) Hereinafter, the present invention will be described in more detail by way of examples. However, these examples are given to merely illustrate the invention and are not intended to limit the scope of the invention thereto.

Example 1: Preparation of Chalcogen-Containing Compound (V.SUB.0.8.Na.SUB.0.2.Sn.SUB.3.2.Pb.SUB.0.8.Bi.SUB.2.Se.SUB.6.95.Te.SUB.0.05.)

(12) The respective powders of Na, Sn, Pb, Bi, Se, and Te, which are high purity raw materials, were weighed at a molar ratio of 0.2:3.2:0.8:2:6.95:0.05 in a glove box and placed in a graphite crucible, and then charged into a quartz tube. The inside of the quartz tube was evacuated and sealed. Then, the raw materials were kept at a constant temperature in an electric furnace at 750° C. for 24 hours, and slowly cooled at room temperature.

(13) Thereafter, heat treatment was carried out at a temperature of 640° C. for 48 hours. The quartz tube in which the reaction progressed was cooled with water to obtain an ingot. The ingot was finely pulverized to a powder having a particle size of 75 μm or less, and sintered according to a spark plasma sintering method (SPS) at a pressure of 50 MPa and a temperature of 620° C. for 10 minutes to prepare a chalcogen-containing compound of V.sub.0.8Na.sub.0.2Sn.sub.3.2Pb.sub.0.8Bi.sub.2Se.sub.6.95Te.sub.0.05.

Example 2: Preparation of Chalcogen-Containing Compound (V.SUB.0.8.Na.SUB.0.2.Sn.SUB.3.2.Pb.SUB.0.8.Bi.SUB.2.Se.SUB.6.6.Te.SUB.0.4.)

(14) A chalcogen-containing compound of V.sub.0.8Na.sub.0.2Sn.sub.3.2Pb.sub.0.8Bi.sub.2Se.sub.6.6Te.sub.0.4 was prepared in the same manner as in Example 1, except that the respective powders of Na, Sn, Pb, Bi, Se, and Te, which are high purity raw materials, were mixed at a molar ratio of 0.2:3.2:0.8:2:6.6:0.4 in a glove box.

Example 3: Preparation of Chalcogen-Containing Compound (V.SUB.0.8.Na.SUB.0.2.Sn.SUB.3.2.Pb.SUB.0.8.Bi.SUB.2.Se.SUB.6.2.Te.SUB.0.8.)

(15) A chalcogen-containing compound of V.sub.0.8Na.sub.0.2Sn.sub.3.2Pb.sub.0.8Bi.sub.2Se.sub.6.2Te.sub.0.8 was prepared in the same manner as in Example 1, except that the respective powders of Na, Sn, Pb, Bi, Se, and Te, which are high purity raw materials, were mixed at a molar ratio of 0.2:3.2:0.8:2:6.2:0.8 in a glove box.

Comparative Example 1: Preparation of Chalcogen-Containing Compound (V.SUB.0.8.Na.SUB.0.2.Sn.SUB.4.Bi.SUB.2.Se.SUB.7.)

(16) A chalcogen-containing compound V.sub.0.8Na.sub.0.2Sn.sub.4Bi.sub.2Se.sub.7 was prepared in the same manner as in Example 1, except that the respective powders of Na, Sn, Bi, and Se, which are high purity raw materials, were mixed at a molar ratio of 0.2:4:2:7 in a glove box.

Comparative Example 2: Preparation of Chalcogen-Containing Compound (NaSn.SUB.3.95.Pb.SUB.0.05.Bi.SUB.2.Se.SUB.7.)

(17) A chalcogen-containing compound of NaSn.sub.3.95Pb.sub.0.05Bi.sub.2Se.sub.7 was prepared in the same manner as in Example 1, except that the respective powders of Na, Sn, Pb, Bi, and Se, which are high purity raw materials, were mixed at a molar ratio of 1:3.95:0.05:2:7 in a glove box.

Comparative Example 3: Preparation of Chalcogen-Containing Compound (Na.SUB.0.2.Sn.SUB.4.75.Pb.SUB.0.05.Bi.SUB.2.Se.SUB.7.)

(18) A chalcogen-containing compound of Na.sub.0.2Sn.sub.4.75Pb.sub.0.05Bi.sub.2Se.sub.7 was prepared in the same manner as in Example 1, except that the respective powders of Na, Sn, Pb, Bi, and Se, which are high purity raw materials, were mixed at a molar ratio of 0.2:4.75:0.05:2:7 in a glove box.

(19) Comparative Example 4: Preparation of Chalcogen-Containing Compound (Na.sub.0.2Sn.sub.3.95Pb.sub.0.05Bi.sub.2.8Se.sub.7)

(20) A chalcogen-containing compound of Na.sub.0.2Sn.sub.3.95Pb.sub.0.05Bi.sub.2.8Se.sub.7 was prepared in the same manner as in Example 1, except that the respective powders of Na, Sn, Pb, Bi, and Se, which are high purity raw materials, were mixed at a molar ratio of 0.2:3.95:0.05:2.8:7 in a glove box.

Reference Example: Preparation of Chalcogen-Containing Compound (V.SUB.0.8.Na.SUB.0.2.Sn.SUB.3.2.Pb.SUB.0.8.Bi.SUB.2.Se.SUB.7.)

(21) A chalcogen-containing compound of V.sub.0.8Na.sub.0.2Sn.sub.3.2Pb.sub.0.8Bi.sub.2Se.sub.7 was prepared in the same manner as in Example 1, except that the respective powders of Na, Sn, Pb, Bi, and Se, which are high purity raw materials, were mixed at a molar ratio of 0.2:3.2:0.8:2:7 in a glove box.

Experimental Example

(22) 1. Phase Analysis According to XRD Pattern

(23) For the chalcogen-containing compounds in a powder state just before the sintering step in Examples 1 to 3, Comparative Examples 1 to 4, and Reference Example, X-ray diffraction analysis was carried out, and the results are shown in FIG. 2 and FIG. 3. In addition, the sintered body finally produced through the sintering step in Examples 1 to 3, Comparative Example 1, and Reference Example was gradually cooled from about 620° C. to 300° C. and then cooled again to room temperature (25° C.). Then, the sintered body was maintained in the air atmosphere for 15 days, and X-ray diffraction analysis of each sintered body was performed. The results are shown in FIG. 4.

(24) First, referring to FIG. 2, the chalcogen-containing compounds of Examples 1 to 3, Comparative Example 1, and Reference Example were confirmed to have the same crystal lattice structure as that of Sn.sub.4Bi.sub.2Se.sub.7 which is conventionally known to have a face-centered cubic lattice structure at a high temperature. From these results, it was confirmed that the chalcogen-containing compounds of Examples 1 to 3, Comparative Example 1, and Reference Example all had a face-centered cubic crystal lattice structure.

(25) On the other hand, referring to FIG. 3, the compounds of Comparative Examples 2 to 4 are chalcogen-containing compounds having various compositions without a vacancy site, particularly, Comparative Example 3 is a case where the content of Sn exceeds 4, and Comparative Example 4 is a case where the content of Bi exceeds 2. Although the chalcogen-containing compounds of Comparative Examples 2 to 4 were synthesized in the same manner as in Example 1, it was confirmed that in Comparative Examples 2 to 4, the vacant sites were all filled and a single phase having a face-centered cubic lattice structure could not be formed as the content of Na, Sn, or Bi increased. From these results, it was confirmed that when vacant sites exist, excellent phase stability is maintained even at a relatively low temperature.

(26) Further, referring to FIG. 4, it was confirmed that the compounds of Examples 1 to 3, Comparative Example 1, and Reference Example maintained the face-centered cubic lattice structure without generation of secondary phases and exhibited excellent phase stability, when left at a relatively low temperature. From these results, it was confirmed that the compounds of Examples 1 to 3, Comparative Example 1, and Reference Example exhibited excellent phase stability even at a relatively low temperature.

(27) 2. Rietveld Refinement Calculation

(28) The lattice parameter and the Rietveld refinement were calculated for each of the chalcogen-containing compounds in a power state of Examples 1 to 3, Comparative Example 1, and Reference Example using the TOPAS program, and the results are shown in Table 1 below.

(29) TABLE-US-00001 TABLE 1 Comparative Reference Exam- Exam- Exam- Example 1 Example ple 1 ple 2 ple 3 Lattice 5.9645 5.9816 5.9817 5.9827 5.9950 parameter (Å) Vacancy (0, 0, 0) 0.1116 0.1143 0.1134 0.1137 0.1163 occupancy Na (0, 0, 0) 0.0286 0.0286 0.0286 0.0286 0.0272 occupancy Sn (0, 0, 0) 0.5741 0.4571 0.4577 0.4571 0.4569 occupancy Bi (0, 0, 0) 0.2857 0.2857 0.2857 0.2857 0.2856 occupancy Pb (0, 0, 0) — 0.1143 0.1146 0.1149 0.1141 occupancy Se (0.5, 0.5, 0.5) 1 1 0.9929 0.9303 0.8933 occupancy Te (0.5, 0.5, 0.5) — — 0.0090 0.0530 0.1190 occupancy Rwp (weighted 5.01 6.34 6.59 5.60 6.03 pattern R)

(30) Referring to Table 1, it was confirmed that as the content of Te substituted in the site of Se was increased in the face-centered cubic structure, lattice parameter value gradually increased. That is, the lattice parameters were increased in the order of Example 3>Example 2>Example 1>Reference Example. From this result, it could be seen that Te having a larger atomic radius was substituted well for Se. Further, when Pb was substituted in the site of Sn, the lattice parameters of Reference Example were increased relative to Comparative Example 1. From this result, it could be seen that Pb having a larger atomic radius was substituted well for Sn.

(31) On the other hand, it was confirmed that vacancy (V), Na, Sn, Pb, and Bi were randomly distributed in the site of (x, y, z)=(0, 0, 0), and in the case of Se and Te, they were randomly distributed in the site of (x, y, z)=(0.5, 0.5, 0.5). Further, it was confirmed that each composition contained in the chalcogen-containing compound was very similar to each molar ratio of Na, Pb, Sn, Bi, Se, and Te, which are high purity raw materials.

(32) 3. Temperature Dependence of Electrical Conductivity

(33) For the chalcogen-containing compound samples prepared Examples 1 to 3, Comparative Example 1, and Reference Example, the electrical conductivity was measured according to the temperature change, and the results are shown in FIG. 5. The electrical conductivity was measured at a temperature range of 50° C. to 400° C. by a four-probe DC method using a measuring device LSR-3 (manufactured by Linseis), which is a resistivity measuring device.

(34) Referring to FIG. 5, when a part of Sn is substituted with Pb, Pb substituted at the site of Sn caused a change in the electronic structure of the chalcogen-containing compound, thereby increasing the electrical conductivity (Comparative Example 1 and Reference Example). However, as a part of Se was replaced with Te, the electrical conductivity decreased as the content of the substituted Te increased (Reference Example>Example 1>Example 2>Example 3). These results indicate that as the content of Te increases, the electron scattering due to the mass difference between Se and Te, which forms the chalcogen-containing compound frame, is further increased, and as a result, the electrical conductivity is reduced, and further, the electron concentration decreased as the content of Te increased. This can be confirmed from the fact that the bipolar effect of the Seebeck coefficient in FIG. 6 becomes larger as the Te content increases. Due to the reduction of electron mobility and electron concentration resulting from such scattering, the electrical conductivity of the chalcogen-containing compounds decreased gradually when the content of Te increased.

(35) 4. Temperature Dependence of Seebeck Coefficient

(36) For the chalcogen-containing compound samples prepared in Examples 1 to 3, Comparative Example 1, and Reference Example, the Seebeck coefficient (S) was measured according to the temperature change, and the results are shown in FIG. 6. The Seebeck coefficient was measured in a temperature range of 50° C. to 400° C. by using a measuring device LSR-3 (manufactured by Linseis) and applying a differential voltage/temperature technique.

(37) As shown in FIG. 6, Examples 1 to 3, Comparative Example 1, and Reference Example showed a negative Seebeck coefficient. From this fact, it was confirmed that the main charge carrier of the material is electrons, which exhibited characteristics as an N-type semiconductor material.

(38) On the other hand, in Reference Example in which a part of Sn is substituted with Pb, despite the increase in electrical conductivity as compared with Comparative Example 1, the Seebeck coefficient also increased. In Examples 1 to 3, in which a part of Se was substituted with Te and the content thereof was increased, the Seebeck coefficient values were increased as compared with Comparative Example 1 and Reference Example. From these results, it was confirmed that Examples 1 to 3 have excellent electrical properties.

(39) 5. Temperature Dependence of Power Factor

(40) For the chalcogen-containing compound samples prepared in Examples 1 to 3, Comparative Example 1 and, Reference Example, the power factors were calculated according to the temperature change and are shown in FIG. 7.

(41) The power factor is defined as power factor (PF)=σS.sup.2, and was calculated using the values of σ (electrical conductivity) and S (Seebeck coefficient) shown in FIG. 5 and FIG. 6.

(42) As shown FIG. 7, it was confirmed that the chalcogen-containing compounds of Examples 1 to 3 and Reference Example in which Pb was substituted for a part of Sn exhibited excellent power factors as compared with Comparative Example 1. In addition, Examples 1 to 3, in which Te was further substituted for a part of Se, showed a high power factor at 50° C. to 250° C. as compared with Reference Example.

(43) 6. Temperature Dependence of Total Thermal Conductivity and Lattice Thermal Conductivity

(44) For the chalcogen-containing compound samples prepared in Examples 1 to 3, Comparative Example 1, and Reference Example, the total thermal conductivity and the lattice thermal conductivity were measured according to the temperature change, and the results are shown in FIG. 8 and FIG. 9, respectively. In the measurement of the thermal conductivity, first, the thermal diffusivity (D) and the thermal capacity (Cp) were measured by applying a laser scintillation method and using an LFA457 instrument (manufactured by Netzsch) which is a device for measuring the thermal conductivity. The thermal conductivity (k) was calculated by applying the measured value to the equation of “thermal conductivity (k)=DρC.sub.p (ρ is the density of sample measured by Archimedes method)”.

(45) In addition, the total thermal conductivity (k=k.sub.L+k.sub.E) is divided into the thermal conductivity (k.sub.E) calculated according to the lattice thermal conductivity (k.sub.L) and the Wiedemann-Franz law (k.sub.E=LσT), wherein the value calculated from the Seebeck Coefficient versus temperature was used as the Lorentz number (L).

(46) Referring to FIG. 8, Examples 1 to 3, in which Pb and Te were simultaneously substituted, showed low thermal conductivity as compared with Comparative Example 1 and Reference Example, and in particular, as the Te content increased, the total thermal conductivity decreased.

(47) On the other hand, referring to FIG. 9, all the chalcogen-containing compounds of Examples 1 to 3, Comparative Example 1, and Reference Example exhibited low lattice thermal conductivities, and this is due to phonon scattering by the vacancy (V) of the face-centered cubic lattice structure. In the case of Reference Example in which Pb is substituted exhibited lower lattice thermal conductivity as compared with Comparative Example 1. In the case of Examples 1 to 3 in which Te is further substituted together with Pb substitution, they exhibited lower lattice thermal conductivity as compared with Reference Example. This is because the phonon scattering effect due to the mass difference between Pb and Sn, and between Se and Te, simultaneously functioned. Particularly, in the case of Example 3, the lattice thermal conductivity at 250° C. was found to be very low, at about 0.54 W/mK.

(48) 7. Temperature Dependence of Thermoelectric Performance Index (ZT)

(49) For the chalcogen-containing compound samples prepared in Examples 1 to 3, Comparative Example 1, and Reference Example, the thermoelectric performance index was calculated according to temperature change, and the results are shown in FIG. 10.

(50) The thermoelectric performance index (ZT) is defined as ZT=S.sup.2σT/k, and was calculated by using the values of S (Seebeck coefficient), σ (electrical conductivity), T (absolute temperature), and k (thermal conductivity) obtained in Experimental Examples.

(51) Referring to FIG. 10, it was confirmed that Examples 1 to 3 exhibit thermoelectric performance indices that are applicable to thermoelectric materials. In Comparative Example 1 and the reference example, ZT was increased by substituting Pb for a part of the Sn site. In Examples 1 to 3, Te was substituted for a part of Se, and as the substitution amount of Te increased, the ZT value was further increased. In particular, at 250° C., the ZT value of Example 3 increased by 220% and 40%, respectively, compared to the ZT values of Comparative Example 1 and the reference example.